I agree with CptSandL ,this is what the book says.The problem with the SB on the 737 it's that they can produce some strong vibrations at full flight detent position.These vibrations will be greatly reduced with the SB just bellow flight detent.
On the NG this was a problem from the beginig ,causing some LCO,at speeds greater than 300kts.They did some modifications on the stabiliser,and this restriction was lifted,but we still have a warning in the FCOM ,not to use the SB at speeds greater than 320 kts on the NG.
Indeed,the 737 has very good aerodinamics,and sometimes flyes like a glider,the speed refusing to go down.It is easier to reduce speed during level flight,especially with a tail wind.
The NG it's even more slippery than the Classic.
I use speed no more than 300 kt during descent,this will give less vibrations at flight detent SB,if not required otherwise by the atc.Also the turbulance speed for the 737 is 280 kts,easier and comfy to go there from 300 than from 320 kts.
Brgds Alex